Coating compositions are commonly applied to a substrate by passing them under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid coating. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the coating composition into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the coating composition and high-velocity air outside of the nozzle to cause atomization. Auxiliary air streams modify the shape of the spray. The coating composition flows through the orifice in the spray nozzle at low pressure, typically less than 18 psi. Air spray is used to apply high quality coatings because of its ability to produce a fine droplet size and a "feathered" spray, that is, the spray has a uniform interior and tapered edges. Such a feathered spray is particularly desirable so that adjacent layers of sprayed coating can be overlapped to form a coating with uniform thickness. However, because of the high air volume that is used, air spray deposits the coating inefficiently onto the substrate, that is, it has low transfer efficiency, which wastes coating.
Airless spray uses a high pressure drop across the orifice to propel the coating composition through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. The momentum of the spray carries the droplets to the substrate. Spray pressures typically range from 700 to 5000 psi. The spray tip is contoured to modify the shape of the spray, which is usually a round or elliptical cone or a flat fan. Because no compressed air is used, airless sprays deposit the coating composition more efficiently onto the substrate, that is, it has higher transfer efficiency, than air sprays. However, its use is generally limited to applying low quality coatings because it characteristically does not provide a "feathered" spray or fine atomization. Conventional airless spray techniques are known to typically produce coarse droplets and defective spray fans. These deficiencies become less severe if a relatively large concentration of organic solvent is used to lower the atomization viscosity. However, the deficiencies become much more severe if less solvent is used and atomization viscosity is increased in order to reduce solvent emissions. The spray characteristically forms a "tailing" or "fishtail" spray pattern, because surface tension gathers more liquid at the edges of the spray fan than in the center. This produces coarsely atomized jets of coating and a non-uniform spray pattern, which makes it difficult to apply a uniform coating. Airless sprays are generally angular in shape and have a fan width generally equal to the fan width rating of the spray tip being used.
Air-assisted airless spray combines features of air spray and airless spray, with intermediate results. It uses both compressed air and high pressure drop across the orifice to atomize the coating composition and to shape the spray, typically under milder conditions than each type of atomization is generated by itself. The air assist helps to atomize the liquid film and to smooth out the spray to give a more uniform fan pattern. Generally the compressed air pressure and air flow rate are lower than for air spray. Liquid spray pressures typically range from 200 to 800 psi. However, like an air spray, air-assisted airless spray requires a relatively low viscosity, typically below 100 centipoise, and therefore uses a high concentration of organic solvents. The compressed air usage also typically produces lower transfer efficiency than with airless spray.
Airless spray and air-assisted airless spray can also be used with the coating composition heated or with the air heated or with both heated. Heating reduces the viscosity of the coating composition and aids atomization.
A problem generally associated with orifice spray techniques, but more particularly with airless spray and air-assisted airless spray, is entrapment of fine air bubbles within the coating during application, which produces an inferior coating. It is particularly troublesome in clear coatings, because light reflected from the air bubbles gives the coating a white hazy appearance, but it is troublesome in pigmented coatings as well. The bubbles cause poor coating appearance, such as by distorting the surface, and cause poor coating performance, such as by decreasing corrosion protection and surface hardness. The bubbles may also become exposed through the surface due to surface wear from sanding or buffing operations and thereby render the coating unacceptable. In baked coatings, the bubbles serve as nucleation sights for solvent evaporation during baking and thereby can cause severe solvent popping in the coating. Sometimes, during heating, the bubbles expand and migrate to the surface, but in doing so they often form craters and tiny pits in the coating surface. This reduces coating gloss and distinctness of reflected image.
Without wishing to be bound by theory, air entrapment during spray application of a coating is believed to occur by more than one mechanism, depending upon the properties of the spray and coating. One mechanism is a high velocity droplet penetrating into the coating interior and forming a channel filled with air; the air becomes trapped in the coating film when the coating surface flows together or another droplet is deposited on top of it. This is consistent with the observation that sometimes air entrapment does not occur during application until the coating reaches a certain thickness. A coating with low viscosity would be expected to be susceptible to air entrapment by this mechanism. Higher coating viscosity would be expected to reduce droplet penetration, but the viscosity must remain low enough for rapid reflow to give a smooth coating. Another mechanism would be expected to occur with highly viscous coatings or with coatings that wet the substrate surface poorly. Under these conditions, the droplets tend to remain spherical for a period of time after impact instead of immediately spreading out and coalescing with their neighbors. Therefore, the droplets stack on top of each other and air becomes trapped in void spaces between them. This would also be expected to occur with normal viscosity coatings when droplets are deposited very rapidly on top of each other. This is consistent with the observation that air entrapment often occurs more readily when the spray builds up coating thickness very rapidly, such as when the traverse speed is low or the spray is very concentrated. Air entrapment sometimes occurs in streaks from concentrated portions of a non-uniform spray, which would be expected to create a churning action that would entrap air.
Entrapped air bubbles in a coating are generally smaller than the spray droplets that deposit the coating. Typically they are individual spherical air bubbles that lie in the interior of the coating film. Generally they have a diameter less than about 30 microns, although larger bubbles can also occur, particularly in thick coatings. The bubbles can be seen individually through a microscope or collectively by the hazy appearance that they give to a clear coating.
Miyamoto, in U.S. Pat. No. 4,842,900, issued Jun. 27, 1989, discloses a method and apparatus for using curtain coating or extrusion coating to apply a liquid film of a coating composition onto a traveling web in manufacturing photographic film, photographic printing paper, magnetic recording tape, adhesive tape, pressure sensitive recording paper, offset paper, and the like. The liquid film is formed by causing coating composition to flow in a single layer or a plurality of layers out of a die through a slit or slits. Just before the liquid film contacts the traveling web, the air entrained with the web is replaced by a gas which is highly soluble in the coating composition. The preferred gas is carbon dioxide. The speed of the traveling web can be significantly increased because the entrained bubbles of soluble gas are dissolved in the time of one-hundredth of a second or less. In the example disclosed therein, the speed of the traveling web was increased from 65 to 200 meters per minute.
Prior to the present invention, there has been no effective way to remove entrapped bubbles from a spray applied coating other than to try to promote their migration to the surface, followed by breakage of the surfaced bubble. To this end, various surface active agents or surfactants have been used in coating formulations, as is well known to those skilled in the art. But these surface active agents, which function effectively as defoamers in breaking foams and surface bubbles, and which also aid surface flow to prevent cratering, have proven to have limited effectiveness as air release agents, that is, in promoting migration of entrapped bubbles through the interior of the coating to the coating surface and thereby eliminating the air entrapment problem. The effectiveness of the surface active agent is also highly dependent upon properly matching the properties of the agents with the properties of the coating formulation, which usually must be determined by trial and error. Because many different surface active agents have been developed, this can be a time consuming and costly process, particularly if several coatings are applied, such as on a paint line where color change is employed. Moreover, because surface active agents are used to treat a variety of coating application problems, such as wetting, cratering, fisheyes, foaming, and pigment dispersion, the appropriate amount of surface active agent for one problem is often not the proper amount for another problem, so a compromise amount must be used. Therefore, it is desirable to remove air entrapment as a problem to be treated using surface active agents so that other problems may be more effectively treated. Furthermore, as aforementioned, migration of bubbles to the surface often leaves tiny pits on the hardened coating, which greatly reduces coating quality such as by reducing gloss and distinctness of reflected image.
Due to the high viscosity coating compositions that are typically utilized in the inventions described in the aforementioned related patents and patent application, air entrapment may be particularly noticeable. Specifically, prior to the inventions described in the aforementioned related patents and patent application, the liquid spray application of coatings, such as paints, lacquers, enamels, and varnishes, was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution from coating operations. Therefore, great emphasis has been placed on the development of new coating technologies that diminish the emission of organic solvent vapors.
Such a new coating technology is discussed in the aforementioned related patents and patent application, particularly U.S. Pat. No. 4,923,720, which teach, among other things, the utilization of supercritical fluids or subcritical compressed fluids, such as carbon dioxide or nitrous oxide, as viscosity reducing diluents in highly viscous organic solvent-borne coating compositions and/or highly viscous non-aqueous dispersions coating compositions to dilute these coatings to application viscosity required for liquid spray techniques.
As used herein, it will be understood that a "supercritical fluid" is a material which is at a temperature and pressure such that it is at, above, or slightly below its "critical point". As used herein, the "critical point" is the transition point at which the liquid and gaseous states of a substance merge into each other and represents the combination of the critical temperature and critical pressure for a given substance. The "critical temperature", as used herein, is defined as the temperature above which a gas cannot be liquified by an increase in pressure. The "critical pressure", as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the critical temperature.
As used herein, a "compressed fluid" is a fluid which may be in its gaseous state, its liquid state, or a combination thereof depending upon the particular temperature and pressure to which it is subjected upon admixture with the composition that is to have its viscosity reduced and the vapor pressure of the fluid at that particular temperature, but which is in its gaseous state at standard conditions of 0.degree. C. and one atmosphere pressure (STP). The compressed fluid may comprise a supercritical or subcritical fluid.
As used herein, the phrases "coating composition", "coating material", and "coating formulation" are understood to mean conventional coating compositions, materials, and formulations that have no supercritical fluid or subcritical compressed fluid admixed therewith. Also as used herein, the phrases "spray mixture", "liquid mixture", and "admixed coating composition" are meant to include an admixture of a coating, coating material, coating composition, or coating formulation with at least one supercritical fluid or at least one subcritical compressed fluid.
As disclosed in the aforementioned patent applications, it has been discovered that supercritical fluids or subcritical compressed fluids are not only effective viscosity reducing diluents, but they can also remedy the defects of the airless spray process by creating vigorous decompressive atomization by a new airless spray atomization mechanism, which can produce the fine droplet size and feathered spray needed to apply high quality coatings.
In the spray application of coatings using supercritical fluids or subcritical compressed fluids such as carbon dioxide, the large concentration of carbon dioxide dissolved in the coating composition produces a liquid spray mixture that has markedly different properties than conventional coating compositions. In particular, the spray mixture is highly compressible, that is, the density changes markedly with changes in pressure, whereas conventional coating compositions are incompressible liquids when they are sprayed.
Without wishing to be bound by theory, it is believed that vigorous decompressive atomization can be produced by the dissolved carbon dioxide suddenly becoming exceedingly supersaturated as the spray mixture leaves the nozzle and experiences a sudden and large drop in pressure. This creates a very large driving force for gasification of the carbon dioxide, which overwhelms the cohesion, surface tension, and viscous forces that oppose atomization and normally bind the fluid flow together into a fishtail type of spray.
A different atomization mechanism is evident because atomization occurs right at the spray orifice instead of away from it as is conventional. Atomization is believed to be due not to the break-up of the liquid film from shear with the surrounding air but, instead, to the expansive forces of the compressible spray solution created by the carbon dioxide. Therefore, no liquid film is visible coming out of the nozzle.
Furthermore, because the spray is no longer bound by cohesion and surface tension forces, it leaves the nozzle at a much wider angle than normal airless sprays and produces a "feathered" spray with tapered edges like an air spray. This produces a rounded, parabolic-shaped spray fan instead of the sharp angular fans typical of conventional airless sprays. The spray also typically has a much wider fan width than conventional airless sprays produced by the same spray tip. As used herein, the terms "decompressive atomization" and "decompressive spray" each refer to a spray, spray fan, or spray pattern that has the preceding characteristics.
Laser light scattering measurements and comparative spray tests show that decompressive atomization can produce fine droplets that are in the same size range as air spray systems instead of the coarse droplets produced by normal airless sprays. These fine droplets are ideal for minimizing orange peel and other surface defects commonly associated with spray application. This fine particle size provides ample surface area for the dissolved carbon dioxide to very rapidly diffuse from the droplets within a short distance from the spray nozzle. Therefore, the coating contains little dissolved carbon dioxide when it is deposited onto the substrate.
As disclosed in the aforementioned patent applications, coating compositions formulated for spraying with supercritical fluids or subcritical compressed fluids, called coating concentrates, have much less organic solvent content than conventional coatings, in order to reduce air pollution, but typically utilize relatively high molecular weight polymers. Consequently, the coating concentrates have a high viscosity, typically 800 to 3000 centipoise at a temperature of 25.degree. Celsius and atmospheric pressure, which is much higher than normal coating compositions. Because the coating concentrate is applied to the substrate with little dissolved supercritical fluid or subcritical compressed fluid, which is released as gas from the droplets in the spray, the coating is deposited on the substrate with a viscosity that is the same or higher than that of the coating concentrate. This often enables the coating to be applied to final thickness in one application without running or sagging. Therefore, because of the higher coating viscosity, migration of entrapped air bubbles to the surface of the coating is usually much less effective than in conventional coatings.
In contrast, even conventional high-solids coatings have a viscosity that is not much higher than that of low-solids coatings. Typically, high-solids clear coats have viscosities of about 80 centipoise and base coats have viscosities of about 35 centipoise, both at a temperature of 25.degree. Celsius. Even after solvent is lost in the spray, conventional low-solids and high-solids coatings are typically deposited onto the substrate with considerably lower viscosity than the coating concentrates. With conventional low-solids coatings, the coating usually must be applied in several layers to allow excess atomization solvent to evaporate between layers to avoid running and sagging. Conventional high-solids coatings likewise have relatively low deposition viscosity, as evident by the running and sagging problem caused by the low molecular weight polymers used to obtain low atomization viscosity with less solvent.
For these reasons, there is clearly a need for new liquid spray technology that significantly prevents or minimizes air entrapment in coatings. The new technology should generally be applicable to orifice sprays, be applicable to a wide variety of coating formulations and coating materials, be readily implemented, and be environmentally compatible. In particular, it should be compatible with and augment new orifice spray processes that have been developed to use coatings with much less solvent and air toxic materials than conventional coatings and spray processes, in order to significantly reduce air pollution and worker exposure to toxic solvents.